The present invention relates to the art of nuclear magnetic resonance. Particular application is found in conjunction with magnetic resonance imaging in regions of a patient undergoing periodic respiratory movement and description will be made with reference thereto. It is to be appreciated, however, that the present invention is also applicable to monitoring other objects which have moving portions, such as imaging regions of a patient adjacent the heart, regions with periodic cardiovascular expansion and contraction, regions with spasmodic muscular action, and other regions of animate and inanimate objects with moving portions or components.
Heretofore, magnetic resonance imaging included positioning the patient in a strong substantially uniform, longitudinal magnetic field. Magnetic dipoles were caused to precess, generating magnetic resonance signals. Various known techniques have been utilized to select a region of the patient to be imaged and are well known in the art. Magnetic field gradients were applied orthogonally to encode spatial position into a selected slice or planar region of interest. The magnetic field gradients were encoded along one axis giving a frequency which varied linearly in accordance with position and along an orthogonal axis giving a phase that varied linearly in accordance with position.
The resonance signal was collected with each of a plurality of phase encodings and transformed from a frequency and phase domain to an image or spatial domain. In the image domain, each transformed resonance signal produced a view representing the density of resonating nuclei in each pixel or incremental area of the image region. The amplitude of the phase encoding gradient was incrementally increased with equal steps from view to view to collect a plurality of views, commonly on the order of 128 views. The plurality of views were transformed to create an image representing the density of resonating nuclei in each pixel or incremental area of the image region.
When a portion of the patient or object in the image region was moving, the movement tended to cause a blurring in the resultant image. However, in conventional Fourier transform reconstruction imaging, the blur was not limited to the moving portions. Rather, the distortion was spread across the entire picture in the phase encoding direction. This blurring was commonly manifested in the resultant image as "ghost" artifacts or multiple superimposed coherent image replications along the phase encoding direction or as distributed noise. Moreover, there was an effective periodicity between the respiratory movement and phase encoding gradient over the full image collection cycle. The periodicity enhanced multiple superimposed coherent ghosting.
One solution to respiratory motion artifacts was to gate view collection in accordance with the respiratory cycle. That is, data were only generated when the respiratory movement was at some fixed value, usually near its minimum. A first plurality of views were taken starting with the minimum, negative phase encoding gradient and incrementing upward during a first period of minimum respiratory movement. When the respiratory motion increased beyond a preselected value, the phase encoding and data collection were terminated until the next respiratory cycle. Successive pluralities of phase encoded views were taken in each subsequence minimum movement period. This process was continued until data were collected with a maximum, positive phase encoding gradient, commonly 128 views later. Disruption in data collection during periods of greater respiratory movement or a change in the pattern of respiration increased the duration required to generate an image.
A second solution was to reorder the data to reduce the periodicity of the motion. The loss of image resolution and ghosting attributable to respiratory movement has been reduced by negating the effective periodicity of the respiratory motion. Specifically, the phase encoding gradient values were re-ordered such that at the minimum motion portion of the respiratory cycle, the collected view was phase encoded with the minimum or negative most phase encoding gradient; during the maximum respiratory movement, the collected view was phase encoded with a maximum or positive most phase encoding gradient. Commonly, the minimum phase encoding gradient and the maximum phase encoding gradient were encoded at generally the same spatial frequency but with one delayed and the other advanced such that the minimum phase encoding gradient was negative and the maximum phase encoding gradient was positive. Between the minimum and maximum points of motion the phase was encoded in a linearly proportional relationship between phase encoding gradient and rate or degree of respiratory motion. The median degree of motion midway between the extremes was encoded with a central phase encoding, commonly at the lowest spatial frequency. In this manner, the motion-induced artifacts were reduced by converting the periodicity effectively into a single respiratory period.
One of the drawbacks of this re-ordering scheme was that the data collected during the greatest and the least rates of motion were both encoded at the highest spatial frequencies. Collecting the views with intermediate amounts of motion around the central or zero phase encoding caused a lack of symmetry in the collected data. Certain spatial frequency components were selected preferentially as being less affected by motion in an asymmetric fashion. That is, negative high spatial frequencies were less affected while positive high spatial frequencies were more affected. This had an adverse effect on the point spread function and resolution of the resultant image. It was also a probelm when a smoothing filter was applied because this filter rejected the negative highest spatial frequencies which contain the region of minimal motion (i.e. some of the best data is rejected).
Yet another difficulty resided in the one to one mapping by a probability distribution. Also, in certain patterns of breathing which had essentially no periods of rest, this data was not used most effectively. Further, the technique tended to misplace views near the center (i.e., violated the monotonically increasing function aspect) and hence the method was not robust. Because significant motion may have occurred in the region of low spatial frequency where the signal power was the greatest, the image could still have significant motion artifacts.
Memory limitations restrict the taking of multiple average and multiple slice data.
A further disadvantage of the prior art was that any mapping or reordering of the phase encoding with the amplitude of the motion could not correct the motion. Because motion was not eliminated, an inherent loss of resolution resulted.
The present invention contemplates a new and improved centrally ordered or symmetric phase encoding technique which overcomes the above referenced problems and others.